1. Field of the Invention
The present invention relates to an operational amplifier. More particularly, the present invention relates to a control circuit for an operational amplifier for dynamically modulating the driving capability operational amplifier and a method thereof.
2. Description of the Related Art
Operational amplifiers are now commonly applied in various kinds of circuits. For example, operational amplifiers are used in the amplifier circuit for processing large signal amplification or the driving circuit for driving a capacitive load. Furthermore, digital-to-analog converters (DAC) or analog-to-digital converters (ADC) also require operational amplifiers. In addition, because liquid crystal displays (LCD) need to drive a lot of pixel capacitors, a highly efficient and power-saving driving device is desired.
A conventional operational amplifier is biased by a bias unit so that the operational amplifier can provide enough current to drive external loads. In general, the bias unit converts a fixed reference current into a bias signal in the form of voltage or current, and then the bias unit uses the bias signal to bias the operational amplifier. Therefore, once the reference current is fixed, the driving capability of the operational amplifier for driving external loads is also determined.
Under the consideration of the slew rate and the driving capability, the bias voltage applied to the operational amplifier can be designed to be higher so that the output current from the operational amplifier can be larger. Hence, larger output current leads to faster the level transition speed of the signal, and leads to higher slew rate to driving a larger external load. However, larger output current of the operational amplifier will also lead to higher power consumption. Since there is no mechanism for adjusting the current according to the actual need, current is wasted at the input terminal due to the fixed input voltage. This is a serious drawback for a power-saving device.
Under the consideration of the power saving, the bias signal applied to the operational amplifier can be designed to be smaller so that the output current from the operational amplifier can be smaller. Although some power is saved due to a lower operating current, a lower slew rate and a weaker driving capability will be the price.